1. Field of the Invention
This invention relates to medical imaging systems, and more particularly to imaging systems to locate one or more devices in real-time within or near the imaging volume of the system.
2. Discussion of Prior Art
Several methods exist to follow the location of an invasive device within the body. These methods include MR tracking as disclosed in "Tracking System and Pulse Sequences to Monitor the Position of a Device Using Magnetic Resonance", C. L. Dumoulin, S. P. Souza and R. D. Darrow (U.S. Pat. No. 5,307,808) and radio frequency tracking as disclosed in "Tracking System to Follow the Position and Orientation of a Device Using Radio-Frequency Fields", C. L. Dumoulin, J. F. Schenck, and P. B. Roemer (U.S. Pat. No. 5,377,678). Magnetic resonance tracking methods employ the same physical principles to both locate a device and acquire an MR image, and hence provide excellent registration within the context of the MR image. Positional accuracy of radio frequency tracking, on the other hand, can be degraded by the presence of metallic structures near the tracked device. This loss in positional accuracy can be significant and must be addressed if RF tracking systems are to be used with confidence for invasive medical procedures.
An rf tracking system employs low-frequency electromagnetic fields which are broadcast from an array of transmitter coils and detected by one or more pickup coils, or probes. By measuring the strength of the detected magnetic field, in principle, one can determine the location and orientation of the probes.
The presence of metallic structures near the transmit coils can change the magnetic field sensed by the pickup coils. This in turn, creates an error in the computed position and orientation of the pickup coil. Two mechanisms are possible. First, if the metallic structure is non-ferromagnetic, but conducts electricity, eddy currents are created in the structure. These eddy currents create an "image" field which partially cancels the field at the pickup coil. Larger structures, particularly those which permit eddy currents to flow in large loops are more prone to creating eddy currents. In addition, eddy currents will tend to build up and decay with time constants which are determined by the content and geometry of the eddy-current inducing structures. Thus, eddy currents will introduce a time dependency in the detected signal. If desired, this time dependency can be used as an indicator of the presence of eddy currents.
The second mechanism in which metallic structures interact with the excitation coils occurs when ferromagnetic material is brought near the coils. Ferromagnetic material placed near a transmit coil can change the inductance of the coil and thereby change the efficiency with which the coil makes the detected magnetic field.
If an rf tracking system is to be used in an environment with appreciable amounts of nearby metal (e.g. an MR imaging system) then solutions to the problem of eddy currents are needed. If eddy currents are allowed to distort the acquired data, then errors in positional measurements of the tracked device will occur. What is needed is a means to prevent and/or correct positional errors caused by the proximity of metallic structures within the tracking region of an RF tracking system.